whats the difference between plasma membrane and dialysis tubing?

In short, dialysis tubing acts as a permeable membrane but PURELY based on pore size.  Small molecules like water can get through dialysis tubing based on osmosis, but bigger molecules cannot fit.

Plasma membranes on the other hand are highly complex systems that use both active transport and passive transport to get molecules through.  Typically, plasma membranes have a highly nonpolar core of Hydrocarbons (from phospholipids) that allow small nonpolar molecules to get through.   There are also a TON of other proteins that are embedded in the membrane (or travel from side to side) that let certain molecules through.  Some act as channels and some act as carriers that are specific for certain types of molecules.

Basically, dialysis tubing is simple and only allows diffusion based on whether or not the molecule can fit through the pores while plasma membranes are HIGHLY complex and you would have to read a Bio textbook to begin to understand them.